U.S. Pat. Nos. 4,447,470 and 4,487,813 disclose methods for epitaxially growing HgCdTe upon CdTe substrates. The present invention differs from the method described in these two patents in that a strong crystalline support is used, the CdTe substrate is beneficially eliminated by the end of the fabrication process, and a cool-down interdiffusion step is optionally present. The compositional profile of the finished HgCdTe is ungraded when the present invention is used, whereas the prior art layers described in these two patents have a graded composition, as graphically indicated in FIG. 2 herein. In the prior art patents, the atomic fraction (x) of cadmium in the epitaxially grown HgCdTe which is a distance of at least 20% of the HgCdTe layer thickness away from the CdTe substrate varies by no more than 10% as a function of said distance, but it does vary. On the other hand, the compositional profile of the present invention is totally sharp, in that the atomic fraction (x) of cadmium in the epitaxially grown HgCdTe 15 does not vary through the entire layer 15. Thus, there is no CdTe 5 left at all. This is highly desirable, because it sharpens the cut-off of the finished HgCdTe infrared detector as a function of wavelength, and increases the RA (resistance times area) product.
The method described in U.S. Pat. Nos. 4,447,470 and 4,487,813 converts CdTe to HgCdTe by interdiffusion, but in that process the HgCdTe is formed on bulk CdTe (1 mm thick), which can not be completely converted to HgCdTe in a finite period of time. Consequently, there is a Cd concentration gradient in this layer, and thinning by etching will change the x-value of the layer.
U.S. Pat. No. 4,648,917 entitled "Non Isothermal Method for Epitaxially Growing HgCdTe" and having the same inventors and assignee as the present application, discloses a non isothermal method for epitaxially growing HgCdTe upon CdTe substrates, which method differs from that of the present invention in that: crystalline supports are not always used; there is no cool-down interdiffusion step; there is no restriction on the thickness of the CdTe substrate; and the compositional profile of the finished HgCdTe is graded.
U.S. Pat. No. 4,487,640 discloses a method for epitaxially growing HgCdTe onto CdTe substrates, which method differs from that of the present invention in that: (1) It uses two different sources (16 and 20) maintained at different temperatures, whereas the present invention uses one source. (2) It is not a closely-spaced process, whereas the present invention is. As used herein, "closely-spaced" means the HgTe source and CdTe substrate are spaced apart between 0.1 mm and 10 mm. (3) It uses a carrier gas (H.sub.2 +HX), whereas the present invention does not. (4) The source zones (T.sub.2 and T.sub.3 ) are hotter than the substrate zone (T.sub.1 ). (5) It does not vary the temperatures over the course of the growth phase, whereas the present invention sometimes does.
Tufte et al., "Growth and Properties of Hg.sub.1-x Cd.sub.x Te Epitaxial Layers", J. Applied Phys., Vol. 40, No. 11, pp. 4559-4568 (Oct. 1969), describes methods for growing HgCdTe onto CdTe substrates that produce graded compositional profiles, as opposed to the ungraded profiles in the present invention.
U.S. Pat. No. 4,418,096 discloses a method for epitaxially growing HgCdTe onto CdTe substrate, which method differs from that of the present invention in that: (1) It uses dangerous mercury overpressures of between 4 and 50 atmospheres, col. 2, lines 34-36; the present invention does not use mercury overpressure. (2) The finished layers are at least 158 microns thick, whereas the layers 15 of the present invention are no more than 30 microns thick. (3) The processing time is at least 8 days, whereas that of the present invention is no more than 29 hours. (4) The compositional profile is graded. (5) The source and substrate are always at the same temperature with respect to each other; this is not always true in the present invention.
U.S. Pat. No. 3,622,405 relates to the growth of bulk HgCdTe crystals, not epitaxial layers of same, that are annealed by raising them to temperatures in the vicinity of 755.degree. C.
Vohl et al., J. Electronic Materials, Vol. 7, No. 5, pp. 659-678 (1978), discloses a "hot wall" process for making HgCdTe, rather than a closely-spaced process, in which the starting ingredients are elemental Hg, Cd, and Te.
Tennant, W. E., "Recent Developments in HgCdTe Photovoltaic Devices Grown on Alternative Substrates using Heteroepitaxy", International Electron Devices meeting, Technical Digest, Washington, D.C., Dec. 7, 1983, pp. 704-706, uses liquid phase epitaxy to precipitate Hg.sub..7 Cd.sub..3 Te from a tellurium melt onto a CdTe layer grown on sapphire where the CdTe serves as a template for forming the single crystal HgCdTe layer. The present invention completely converts a layer 5 of CdTe grown on a crystalline support 10 to Hg.sub.1-c Cd.sub.x Te having an x-value 0.18 to 0.5. The advantages of the crystalline support to Tennant are higher durability, larger size, higher uniformity, and lower cost. The advantage of crystalline support 10 in this invention is to support a very thin layer 5 of CdTe which is completely converted to HgCdTe having an x-value of choice. The resulting thin HgCdTe layer 15 having a thickness appropriate for the fabrication of IR (infrared) devices is, in turn, supported by the crystalline support 10. This makes it possible to handle the very thin, compositionally uniform HgCdTe layer 15 in a fashion commensurate with the fabrication of IR devices.
The method of Tennant relies on the precipitation from a Te bath of Hg.sub.0.7 Cd.sub.0.3 Te on top of a CdTe/sapphire layer which serves as a template for forming single crystal Hg.sub.0.7 Cd.sub.0.3 Te. In that method, there is some interdiffusion of the precipitated HgCdTe layer and the CdTe template, so that a Cd concentration gradient exist in the deposited layer; and the product is a double layer of HgCdTe on CdTe. Thus, thinning of the layer by etching results in a different average x-value from the unetched layer. The present method converts by interdiffusion the entire thin film 5 of CdTe to HgCdTe, the x-value of which is controlled. Thus, there is no Cd gradient, and a compositionally uniform layer of HgCdTe on sapphire, GaAs or other crystalline support 10 results. Thinning this layer by etching does not cause any change in x-value.
U.S. Pat. No. 4,435,224 describes a method for fabricating HgCdTe which relies on phase segregation, not interdiffusion; and, it does not form a single homogeneous HgCdTe layer. It forms a near homogeneous HgCdTe layer of one composition on top of a second HgCdTe layer on top of a CdTe layer. The second HgCdTe layer has a marked compositional gradient (see FIG. 3 of the patent).